1. Field of the Invention
The present invention relates to a light source apparatus which has a gas discharge tube, such as a deuterium lamp, for use in optical measuring equipment, such as a spectrophotometer, used in a liquid chromatograph detector.
2. Description of the Related Art
A light source apparatus having a gas discharge tube, such as a deuterium lamp, has a trigger power source unit which applies a high voltage between the electrodes of the gas discharge tube for starting electric discharge, and a constant-current power source unit which applies a constant voltage to the gas discharge tube to maintain the electric discharge of the gas discharge tube and constant-current controls an electric current flowing between the electrodes of the gas discharge tube to maintain luminous intensity constant (for instance, see Japanese Unexamined Patent Publication No. 9-210780).
An example of a conventional typical gas discharge tube driving circuit is shown. FIG. 4 is a circuit diagram schematically showing an example of the driving circuit of a deuterium lamp which is a kind of gas discharge tube.
The driving circuit of a deuterium lamp 102 has a trigger power source unit 103 which applies a high voltage for starting electric discharge in the deuterium lamp 102, a heater power source unit 104 which heats a cathode of the deuterium lamp 102, and a constant-current power source unit 105 which applies a constant discharge maintaining voltage to the deuterium lamp 102.
The trigger power source unit 103 has a direct current power source 106 and a charging circuit including a series circuit of a capacitor 108 and a resistor 110, and switches and connects the charging circuit to either the direct current power source 106 or the deuterium lamp 102 by a switch 112.
To apply the constant voltage for maintaining electric discharge after the electric discharge of the deuterium lamp 102, the constant-current power source unit 105 has a constant-current power source 114, a compensation resistor 117, and a diode 118. To prevent the inflow of the high voltage for starting electric discharge from the trigger power source unit 103, the diode 118 is connected so that the cathode thereof is on the deuterium lamp 102 side. The voltage of the constant-current power source unit 105 is lower than the voltage applied from the trigger power source unit 103.
FIG. 4 shows a state of charging the charging circuit, and the switch 112 is connected to the direct current power source 106 side indicated as <OFF>. When electric discharge is started, the switch is connected to the deuterium lamp 102 side indicated as <ON>, the high voltage is applied to the deuterium lamp 102 by an electric charge accumulated in the capacitor 108, and electric discharge occurs between the electrodes of the deuterium lamp 102. After electric discharge is started in the deuterium lamp 102, the switch 112 is switched to the direct current power source 106 side indicated as <OFF> again, and then, only the constant voltage of the constant-current power source unit 105 is continuously applied to the deuterium lamp 102, thereby maintaining the electric discharge in the deuterium lamp 102.
In the driving circuit of the gas discharge tube, the constant-current power source unit 105 typically constant-current controls the deuterium lamp 102 so that the electric current value is 300 mA at a set voltage of about 150 V. On the other hand, the discharge maintaining voltage for maintaining lighting in the deuterium lamp 102 after the deuterium lamp 102 is lit is about 80±10 V regardless of an individual difference. Therefore, when the set voltage of the constant-current power source unit 105 is set to 150 V, a power loss of about 18 to 24 W obtained by multiplying 150−(80±10)=60 to 80 V by 300 mA occurs in the constant-current power source unit 105 after the deuterium lamp 102 is lit. The power loss appears as heat generation. Conventionally, the heat generation amount is heat-released to increase the temperature in the apparatus, thereby causing drift of a detection signal of a detector.
In addition, for some kinds of deuterium lamps, a voltage of e.g., about 200 V is required to be applied from the constant-current power source unit immediately after the start of lighting, but the discharge maintaining voltage thereafter requires only about 90±20 V. In this case, when electric discharge is maintained at 200 V, the power loss that is the heat generation amount from the light source unit becomes larger, with the result that the stability of analyzing accuracy of the apparatus and the drift ability are deteriorated.
To improve the above problems, there is also an apparatus which lowers the generated voltage of the constant-current power source unit to the voltage necessary for maintaining electric discharge after the deuterium lamp is lit. However, as already described, even when the discharge maintaining voltage of the deuterium lamp is e.g., 90 V in the standards, there is actually an individual difference between 70 and 110 V. Therefore, as the voltage necessary for maintaining electric discharge after the deuterium lamp is lit, 110 V which is the highest is required to be considered. Typically, from the viewpoint of reliability, 130 V obtained by adding an allowance of e.g., 20 V to 110 V is set as the discharge maintaining voltage value. Therefore, when the deuterium lamp in which the actual discharge maintaining voltage is lower than 110 V is used, the power loss becomes larger, with the result that the heat release amount from the light source unit is also increased.